1. Field of the Invention
The present invention relates to a printing apparatus and a printing method which performs printing images on print media by using a print head composed of a plurality of print elements, and more specifically, to a printing method for printing so as to complement printing of a printing area to be otherwise printed by the defective print element by using another normal print element, if any of a plurality of print elements becomes defective, as well as a printing apparatus using the printing method.
2. Description of the Related Art
Proposed printing apparatuses that print images on a printing medium such as a sheet of paper or OHP sheets are provided with print heads based on various printing method. The printing method of the print head includes a wire-dot type, a thermal type, a heat transfer type, or an ink-jet type. In particular, the ink jet type receives attention. This is because this method ejects ink directly on a printing surface of print paper and thus is provided at low running costs and enables to print quietly.
Some of the printing apparatuses are of a carriage scanning type in which a carriage provided with a print head is made to move in a horizontal direction substantially parallel to a printing surface of print paper. In such an ink jet printer of the carriage scanning type, after the print head performs printing on one scan printing area of a printing medium by actuating a large number of nozzles provided in the print head on the basis of print information, while scanning the carriage, the printing medium is fed by a distance corresponding to the one scan printing area in a direction perpendicular to a direction in which the carriage progresses. Consequently, the scan and the conveyance of the print medium are alternately repeated in such a manner to perform printing, thus a predetermined image is formed on the printing surface of the print medium.
A large number of nozzles (ejection openings) for ejecting ink droplets are formed in the print head. Ink used to print images on print media is filled in the nozzles. When an image is printed, nozzles corresponding to image data are appropriately selected among nozzles and printing is performed by ejecting ink droplets from these nozzles.
In ink jet printing apparatuses, in recent years, it is to be desired that printing with an increasingly higher quality and resolution can be realized. As means for realizing this request, finer nozzles are used to form images. On the other hand, fine nozzles having a relatively small ejection opening diameter tend to provide ejection failure easily as compared with conventional nozzles having a large ejection opening diameter. For example, dust or ink with an increased viscosity may adhere to the vicinity of the ejection openings to change the amount of ink ejected. In a severe case, the ink may not be ejected.
Further, in a bubble jet (trade mark) type in which electrothermal converters (heaters) are used to generate bubbles in ink to eject the ink from fine nozzles densely arranged, there is a possibility that any of the heaters are disconnected to preclude the ejection of the ink or ink droplets adhere to an ejection opening surface to cover the ejection openings, resulting in precluding the ejection of the ink.
Therefore, printing unstable that come from the ejection failure of the nozzles may be provided, resulting in degrading print images.
In particular, in a serial type-based printer, printing is carried out by scanning the print head. The presence of a nozzle from which ink cannot be ejected may result in forming lines which are not printed along a scan direction in print images. As a result, white lines appear in a print image. The white lines are a contributing factor significantly degrading the print image.
Owing to this problem, if the number of nozzles is increased to several hundreds or thousands in order to improve a print throughput, the probability of occurrence of abnormal nozzles such as non-ejection nozzles which the ink cannot be ejected from the nozzle increases proportionately. Accordingly, it is difficult to obtain defect-free images
A large number of methods as a remedy have been proposed to deal with this situation; these methods include one for detecting various defective print elements and a method for recovering the print head or carrying out printing, on the basis of the results of the detection.
Japanese Patent Application Laid-open No. 61-123545 (1986) discloses a method for printing by using a normal channel to print based on image data for a defective channel in a printing apparatus that carries out one pass printing in which the same image area is printed during one print scan. Also, the above official gazette discloses method for correcting defective channel portion by the normal channel after the paper is fed by a distance corresponding to an integral multiple of one pixel in order to alternative printing such that when the carriage is made to move rightward for printing, normal printing is carried out, on the one hand, when the carriage is made to move leftward, pixels that cannot be printed owing to defective print elements are printed by using other normal print elements.
Japanese Patent Application Laid-open No. 11-077986 (1999) discloses a method for sequentially switching the correction nozzles in consideration of the lifetimes of the correction nozzles for corrective printing, the method in which the frequency of using the correction nozzles is counted and the correction nozzles are switched if the total use frequency counted reaches a predetermined value. With this method, if the alternative printing is carried out in a manner similar to invention disclosed in Japanese Patent Application Laid-open No. 61-123545 (1986), 2-pass printing is substantially performed.
Japanese Patent Application Laid-open No. 11-000988 (1999) discloses a method of controlling printing using a print head having n print elements. With this method, n/m (m is a divisor of the number of nozzles) print elements are set as first print elements used for normal print scans. Further, other n(m−1)/m print elements are set as second print elements not used for normal print scans. Thus, the second print elements are used as alternatives for a printing operation only if any of the first print elements is defective. A precondition in this case is multipass printing in which an image is basically completed in the same image area in m print scanning and paper feeding operations.
Japanese Patent Application Laid-open No. 10-258526 (1998) discloses a method for completely replacing missing data corresponding to one nozzle with data of another nozzle. With this method, an alternative replacing nozzle is then selected in accordance with the position of the defective nozzle identified after a standard print mask is obtained before printing. Subsequently, print data is deleted from mask data corresponding to the defective nozzle and print data deleted then is added to mask data corresponding to the replacement nozzle. This proposal is premised on the multipass printing as in the case of the method disclosed in Japanese Patent Application Laid-open No. 11-000988 (1999).
In Japanese Patent Application Laid-open No. 2000-094662, a proposed method is method for correcting print data of the non-ejection nozzles by using the other N−1 nozzles, even if ink cannot be ejected from one or more of the N nozzles, though in the case of multipass printing a printing per one raster in N pass is completed by using N nozzles during N print scans. That is, it is considered that pixels to be printed by the non-ejection nozzles are complemented by the other normal nozzles so as to prevent pixels to be printed by the non-ejection nozzles from resulting in blank dots.
Japanese Patent Application Laid-open No. 2001-063008 discloses a method of making corrections using a print element placed in parallel with a defective print element in the print scan direction. Specifically, that discloses method for correcting a defective print element produced in a print head from which a black ink is ejected by a print element in a print head from which a cyan, magenta, and yellow inks are ejected, placed in parallel with the black print head.
The above correction methods can be used to improve the degradation of images caused by non-ejection.
However, if corrective printing is carried out using normal nozzles in place of non-ejection nozzles as described above, the endurance lifetimes of the nozzles used for the correction are reduced by a value corresponding to at least the number of times the nozzles have been used for the correction. The lifetimes of nozzles more frequently used for the correction are over earlier in comparison with those of nozzles not used for the correction. Consequently, the nozzles frequently used for the correction may prematurely cause ejection mis-alignment in which an impacting position prematurely deviates from the regular one, irregular ejection in which an amount of ejection varies, or non-ejection.
That is, in view of preventing the degradation of images caused by non-ejection nozzles, it is necessary to correct the defective part by using normal nozzles. However, in view of the lifetimes of normal nozzles used for the correction, every effort should be made to avoid the corrective printing.
Further, visibility of a missing part of an image formed by a non-ejection nozzle varies depending on the position and amount of the missing part. For example, even if a white line corresponding to one nozzle occurs in only one area of the entire image formed, this missing part is not so noticeable. In particular, if the image is formed of small-diameter dots from fine nozzles, the missing part is not substantially noticeable. On the other hand, if two or three white lines are intensively in a relatively narrow image area, they appear as one thick white line, seen from a distance; they may be thus noticeable.
However, since these image missing parts have been uniformly corrected in the past, even parts that are otherwise unperceived as the degradation of image even without corrections are corrected. Consequently, there is a possibility that the lifetimes of normal nozzles wastefully shrink.
This problem also applies not only to ink jet printing apparatuses but also to other printing apparatuses that carry out printing using a plurality of print elements. The finer one print area printed by each print element is, the less noticeable a missing part corresponding to a defective print element is in the entire image if there is only one missing part. On the other hand, in the area intensively having a plurality of missing parts, these missing parts of printing are noticeable, thus missing parts of printing significantly affect the quality of the entire image.